The communications watchdog hit Optus with a $504,000 fine for spamming consumers who had unsubscribed from the telco's texts and emails.
It's the second-largest fine ever issued by the Australian Communications and Media Authority, with the largest being a $510,000 fine paid by Telstra in 2014.
The authority's chair Nerida O'Loughlin said on Friday marketers needed to respect people's wishes to unsubscribe.
Optus was also caught sending people fake bills with no options to unsubscribe.
"Australians find spam infuriating and as a regulator it is something we are actively cracking down on," Ms O'Loughlin said.
"(The fine) reflects the seriousness of breaches made by Optus and its failure to honour its customers' wishes."
The telco will also appoint an independent consultant to make sure it was complying with spam laws.
Ms O'Loughlin said the authority would be watching and could take Optus to court if it did not comply.
Optus regulatory vice president Andrew Sheridan acknowledged the authority's actions and apologised to customers "who received the messages in error".
"We have committed to putting in place enhanced practices and systems to tighten the management of our marketing communications," he said.
The watchdog has raked in more than $1 million in fines over the last 18 months due to the breaking of spam and telemarketing laws.
Telstra's 2014 fine was for failing to connect customers with phone lines on time.
Corporate offenders caught repeatedly breaking spam laws could face penalties of up to $2.1 million a day, the authority said.
Australian Associated Press